The present invention relates to an acoustic surface wave transducer, and in particular, relates to such a device with improved inband frequency characteristics. The present acoustic surface wave transducer is applicable to mechanical bandpass filter, a delay line, convolver, and/or a resonator, in HF, VHF, and/or UHF bands.
FIG. 1 shows a basic structure of a prior acoustic surface wave device, in which the reference numeral 1 is a piezoelectric substrate made of, for instance, Lithium Niobate (LiNbO.sub.3), LiTaO.sub.3, or PZT, 2A and 2B are interdigital electrodes positioned on the surface of the substrate 1, 4 is a high frequency power source coupled with the first interdigital electrode 2A, and 5a and 5b are output terminals coupled with the second interdigital electrode 2B. The interdigital electrodes 2A and 2B are positioned with a predetermined spacing L, and each interdigital electrode is so arranged that each of the fingers overlap with one another by the length (d). Upon application of a high frequency signal to the first transmission interdigital electrode 2A, the mechanical vibration is induced in substrate 1, and that vibration propagates in the substrate 1 in the perpendicular direction to the fingers in the form of an acoustic surface wave. The second reception interdigital electrode 2B located along the path of that acoustic surface wave transduces the mechanical energy in the substrate 1 to electrical energy, and then between an input signal and an output signal, a signal processing is performed, like a delay line, a bandpass filter, an oscillator, a mixer, and/or a convolver. The width of each finger of an interdigital electrode, and the spacing between two fingers may be 1/4 wavelenth, or 1/8 wavelength. As an alternative, a finger of 1/8 wavelength width and a finger of 5/8 wavelength width are arranged alternately. The arrangement of 1/8 wavelength width of fingers with the spacing of 1/8 wavelength is called a split type interdigital electrode.
Many improvements of such an acoustic surface wave device have been proposed. U.S. Pat. Nos. 3,810,257 and 3,886,504 are a few of these. The former discloses a transducer which reduces triple transit signals, and the latter discloses the improvement of the resonator devices.
FIG. 2 shows another prior transducer, in which the overlap length (d) between the two adjacent fingers is not constant, but depends upon each of the fingers. The symbol A shows the propagation of an acoustic surface wave. An interdigital electrode having the non-uniform overlap length (d) is called an apodized electrode, while an interdigital electrode having the equal overlap length is called a regular electrode. A bandpass filter with a regular interdigital electrode provides single hump frequency characteristics as shown in FIG. 3A, where the horizontal axis shows the frequency and the vertical axis shows the response level. On the other hand, a bandpass filter with an apodized interdigital electrode provides the flat frequency characteristics as shown in FIG. 3B.
In the embodiment of FIG. 2, the overlap length between electrode fingers is long at the middle portion of the electrode, and is short at both the end portions of the electrode, that is to say, a strong acoustic wave is generated at the middle portion of the electrode, and a rather weak acoustic wave is generated at the end portions of the electrode. An apodized electrode is preferable for a bandpass filter which has the improved flat frequency characteristics as shown in FIG. 3B. Those flat characteristics are important for a bandpass filter in a television transmission system, which requires the flat characteristics with less than 0.2 dB in the inband frequency characteristics.
The prior transducer of FIG. 2 has at least two mode for improving the characteristics of the filter. One is the presence of an acoustical absorbent 3 made of plastic near the end of the substrate 1 for preventing the effect of the reflection of the wave at the end of the substrate. The second improvement is providing a rough or uneven surface on the bottom of the substrate 1 for scattering the acoustic wave internally of the substrate, to eliminate the effect of internally directed waves.
However, the prior acoustic wave transducers described above have the disadvantage that the amplitude characteristics and the group delay characteristics have a rather large inband deviation D as shown in FIG. 3C. When a bandpass filter is used in a television transmission apparatus, that inband deviation must be less than 0.2 dB in the amplitude characteristics, and less than 20 nS in the group delay characteristics. However, a prior art bandpass filter can provide only 0.6 dB of inband deviation for the amplitude characteristics, and 60 nS for the group delay characteristics. Therefore, when the inband deviation characteristics must be extremely small, a prior acoustic wave filter cannot be utilized; instead, an LC filter has been utilized for that purpose. However, an LC filter has the disadvantages that its size is large, precise adjustment of an inductance and/or capacitance is necessary, and the phase characteristics are deteriorated in the border of the pass band and the attenuation bands.